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make it easier to determine what is a "bad" DCLS value #4

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souders opened this issue Jun 5, 2019 · 3 comments
Open

make it easier to determine what is a "bad" DCLS value #4

souders opened this issue Jun 5, 2019 · 3 comments

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@souders
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souders commented Jun 5, 2019

I implemented the DCLS code provided here and tried it on a few pages. I got values that ranged from 0.5 to 1.5. My initial reaction was wondering what was a good score and what was a bad score. It sounds like DCLS could get very large - in the thousands potentially.

I think it would be better if the score could be on a fixed scale - say 0 to 100. This would let people have a better understanding of whether layout instability was a priority issue. Since it's not a time-based metric, it might be better to invert it so a score of 100 is perfect (like Lighthouse).

(I read the previous discussion about measuring Layout INstability vs Layout STability. Sorry if this rehashes that debate.)

@skobes
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skobes commented Jun 18, 2019

Having an upper bound is a good idea. We actually do limit the CLS score to 10.0 when we report it to UKM (data source for the Chrome User Experience Report). We could impose a similar limit for the Web Perf API. Differentiating beyond 10.0 is probably not useful.

@mmocny
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mmocny commented Jul 6, 2021

I believe we can now close this, since we have published guidelines on Good, Needs Improvement, and Bad CLS scores.

@npm1
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npm1 commented Jul 6, 2021

I think we could add an example with a diagram showing ~half the viewport shifting and explaining what the score would be in that case.

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